The type of allergy skin testing that is performed is called the Prick Technique.  A small puncture in the skin is made through a drop of allergen extract.  If you are allergic, the spot where the allergen touched your skin will swell and itch a bit, and form a hive.  If you are not allergic, the skin will remain normal.  You will be tested with a variety of allergen extracts so the physician can determine exactly what you are allergic to.  The slightly swollen, small hives go away within 30 minutes.